In many digital imaging applications it is convenient to store digital images in a rendered state, i.e. digital images that have been processed for an output display device. For example, digital images rendered for a computer monitor can be displayed directly on a computer monitor and yield visually pleasing results. Similarly digital images rendered for a digital printer can be transmitted directly to the digital printer and produce pleasing photographic prints.
While rendered state digital images are convenient to use with a specific output device, a rendered state digital image prepared for one output device will generally not produce visually pleasing results when displayed or printed by a different type of output device. However, a rendered state digital image prepared for one device can be transformed such that it has been properly rendered for a different output device. This transformation usually includes a minimum of three separate transforms. The first transform changes the relationship of the pixel values through the use of a first tone scale function, i.e. a single valued function that provides an output pixel value for each input pixel value. The second transform involves a color matrix transform, which accounts for the differences in color response of two different imaging devices. The first tone scale function is designed to prepare the image pixel values in such a manner as to minimize the errors when applying the color matrix transform. The last transform is in the form of a second tone scale function that prepares the color transformed pixel values for the intended output device. The color balance, brightness balance, or contrast characteristics of the image data can be adjusted by applying a linear transform either just after the application of the first tone scale or just before the application of the second tone scale function. While the image quality of results obtained with this method of balance and contrast adjustment is better than when operating on the rendered state digital image data directly, there still can be unwanted color casts imparted to the image data most often experienced in the highlight and shadow regions of images. In particular, the color cast and contrast artifacts can arise for digital images that had been previously rendered using a photographic “S” shaped rendering curve.
It is known in the art that video digital images, i.e. digital images prepared to look good on a computer monitor, can be processed to change their overall brightness and color balance. The first tone scale transform takes the form of an exponential equation or power law function designed to mimic the response of the a typical computer monitor. Since for this application the input and output device are the same, a color matrix transform is not required to account for the differences between the color response of input and output devices. The image data are then converted into a logarithmic domain with a tone scale function. Next, the image data is modified with a linear transform tone scale function to change the image brightness and/or color characteristics. Next, the image data are converted from the logarithmic domain back to a linear domain with a tone scale function. The last step is the application of another exponential tone scale function calculated as the inverse of the first exponential tone scale function. This method also can produce unwanted color casts and contrast artifacts in the processed images, particularly if the digital images had been previously rendered using a photographic “S” shaped rendering curve.
In U.S. Pat. No. 5,184,214, Tatsumi describes an apparatus for editing video images on a video processing console and preparing the images for a photographic printer. The described apparatus processes an input image signal from an image signal source, such as a television camera, to produce an output image signal to record an image on an image recording medium. The input image signal is logarithmically converted into an image signal which matches the image output system using a first look-up table which is selected to match the image signal source. The image signal is then modified with a linear transform to adjust the color and brightness image characteristics. Next, a second look-up table, which is selected to match the image recording medium, is applied to the image signal. Three exponential brightness parameters are determined which form three separate exponential color functions. These exponential color functions are applied to the image signal to achieve an overall desired tone scale. While the method disclosed by Tatsumi is designed to work for video images and for scanned film images, it was not designed to handle digital images that have been previously rendered using a photographic “S” shaped rendering curve.
It is also known in the art that color transparency material can be scanned by a photographic film scanner, modified for its color and brightness balance characteristics and prepared for an output device. The scanned digital image is processed by a first tone scale function designed to have a shape that is the inverse response of the “S” shaped response of the color transparency material. The image data are then modified with a linear transform that can modify the color balance, brightness balance, and contrast image characteristics. The image data are then prepared for display on a computer monitor or digital printer. First, a color transform in the form of a three by three matrix is applied to the image data. The color transform accounts for the differences in the color response of the color transparency material and the intended output device. Next, the image data is modified with a photographic “S” shaped second tone scale function for pleasing photographic results. When processing the image data for a computer monitor, a third tone scale function is applied. This third tone scale function prepares the image data for the computer monitor using an exponential formula designed to be the inverse response of the computer monitor. When processing the image data for a digital printer, the third tone scale function is calculated based on the response of the digital printer. While good photographic image balance and contrast results can be obtained with this method for a particular type of source material and particular output device, it is not designed to adjust the balance and contrast characteristics of a digital image wherein the enhanced digital image is in the same color metric as the original digital image.